There has been tremendous progress in the past several years in the field of diode-pumped frequency-doubled solid state lasers. A variety of applications in optical data storage, reprographics and medical instrumentation have fueled the rapid development of these devices. There have been major advances in both the efficiency of nonlinear conversion as well as the elimination of the mode-competition instability associated with efficient harmonic conversion.
One particularly useful diode-pumped solid-state laser is a micro-laser. A micro-laser is a monolithic or composite cavity, diode-pumped solid-state laser having relatively short cavity and having cavity forming reflective surfaces on opposite ends of the lasant material or an adjacent material.
U.S. Pat. No. 4,847,851 to Dixon discloses a compact, diode-pumped, solid-state laser wherein the diode pump is butt-coupled or close-coupled to a laser gain material which absorbs 63% of the optical pumping radiation within a pathlength of less than 500.mu.. Optical lenses were not needed for coupling.
J. J. Zayhowski and A. Mooradian, "Single-frequency Microchip Nd Lases," Optics Letters, Vol. 14, No. 1, pp. 24-26 (Jan. 1, 1989), have reported the construction of single-frequency microchip lasers which use a miniature, monolithic, flat-flat, solid-state cavity (e.g., 730 micron long cavity) whose mode spacing is greater than the gain bandwidth of the gain medium; and which are longitudinally pumped with the close-coupled, unfocused output of a laser diode. Mooradian has also disclosed in U.S. Pat. No. 4,860,304 a micro-laser employing a gain medium made from a stoichiometric laser material, such as Nd pentaphosphate, and having a cavity length in the range of 10 to 100 .mu.m.
Frequency doubling or second harmonic generation (SHG) uses a non-linear optical crystal to produce laser light having a wavelength of about one half of a predetermined fundamental wavelength. One common or conventional SHG method is intracavity doubling using KTP (i.e., potassium-titanyl-phosphate or KTiOPO.sub.4) as a nonlinear crystal. Up to 180 mW of 532 nm radiation have been obtained in this way with a longitudinally disposed, diode laser pumped Nd:YAG laser. Conventional SHG methods require either additional (i.e., lossy) optical elements in the laser cavity or an external (i.e., frequency tunable) resonator. The conversion of optical radiation at one frequency into optical radiation of another frequency by interaction with a nonlinear optical material within an optical cavity is disclosed in U.S. Pat. No. 4,933,947 to D. W. Anthon and D. L. Sipes and entitled "Frequency Conversion of Optical Radiation". A diode pumped laser having a harmonic generator is disclosed by Robert Byer, G. J. Dixon and T. J. Kane in U.S. Pat. No. 4,739,507 and in an article by Byer, "Diode Laser-Pumped Solid-State Lasers," Science, Vol. 239, Feb. 12, 1988, page 745.
One simple method for SHG of a diode pumped laser uses self-doubling laser materials, like Nd:MgO:LiNbO.sub.3 or NMLN [See T. Y. Fan et al, J. Opt. So. Am. (B), 3, 140 (1980)], or lithium niobate (Li NbO.sub.3) doped with a rare earth, such as thulium, or the substituted stoichiometric neodymium compound, neodymium yttrium aluminum borate (NYAB) or Nd:YAB or Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4. Dorozhkin et al, Sov. Phys. Lett. 7, 555 (1981) and J. T. Lin, Lasers and Optronics, 8 (7), 61 (1989). NYAB is self-doubling for Type-I harmonic generation. Other self-doubling materials include Nd:La B Ge O.sub.4 and Cr:KTP.
Self-frequency doubling using NYAB pumped with flashlamps has been reported. V. G. Dimitriev et al, "Simultaneous Emission at the Fundamental Frequency and the Second Harmonic in an Active Nonlinear Medium: Neodymium Doped Lithium Metaniobate," Soviet Technical Physics Letters, Vol. 5 (11), page 590 (1979). Red light at 660 nm was obtained from 1320 nm optical pumping. The first dye-laser pumped NYAB green laser was reported by Lu in 1986. Baosheng Lu et al, Chinese Physics Letters, 3:413.
The characteristics of NYAB as a diode pumped laser material have been studied. Wang and Stone, Topical Meeting on Advanced Lasers, Session TuB4, Mar. 6, 1990. Wang and Stone used a (3.times.3.times.4) mm crystal of NYAB which was pumped by a GaAlAs laser diode array having 500 mW of output power, a 200 mm collimating lens and a beam conditioning lens. An external output coupling mirror, located 7.5 cm from the front facet of the crystal, was also used.
Recently, diode laser pumped CW operation of NYAB has been reported. Schutz and Wallenstein, "Self-frequency doubling Nd:YAB laser pumped by a diode laser", May 23, 1990, paper CWC4, CLEO-90, Anaheim Calif. In that device, the laser resonator contained only a coated NYAB crystal and an output coupler. With a 1 Watt diode laser as a pump source, a 25 mm long laser resonator produced 10 mW of 532 nm radiation. With optimized output coupling for the fundamental, 180 mW was produced at a wavelength of 1.064 .mu.m. More recently it has been reported that a U.S. patent has been filed for an apparatus which was invented by J. T. Lin and B. Lu and which uses an optical fiber to couple the output from a 1-W diode laser into a (3.times.3.times.3) mm NYAB plano-convex crystal to produce 80 mW green output. J. T. Lin "Doubled Jeopardy: The Blue-Green Race's New Players", Lasers and Optronics, December 1990, page 34. Dual high reflectors were used on one side of the crystal to prevent a loss of harmonic output back towards the pump source. In addition, the opposite side was coated for high transmission at the wavelength of the harmonic. Moreover, no attempt was made to control the coating phase which is critical for an enhancement of the SHG output.
A U.S. patent application, filed on 4/30/90 under Ser. No. 07/516,459, which is entitled "Internally-Doubled, Composite-Cavity Laser" and which is assigned to the assignee of the present invention, describes a composite cavity micro-laser which, in one embodiment, comprises a gain medium, a thin etalon of a nonlinear crystal and a waveplate for polarization control. The effective nonlinearity of the etalon was increased substantially by coating its surfaces to form a harmonic sub-resonator. In that micro-laser, the nonlinear crystal and gain medium were separate elements which were located adjacent to each other. Further size reductions might be possible if the gain medium and the nonlinear crystal were combined by using a self-frequency doubling (SFD) crystal, such as NYAB. However, others have suggested that resonant enhancement of second harmonic generation in NYAB should be impossible because of the high absorption at 522 nm. Schutz, "Miniature Self-Frequency-Doubling CW Nd:YAB Laser Pumped by a Diode-Laser," Optics Communications, Vol. 77(2.3), June 15, 1990, page 221.
Thus, although the practicality of self-doubling laser materials is recognized, a small, close-coupled, diode-pumped, solid-state, self-doubling micro-laser has yet to be realized. Moreover, there is reason to believe that satisfactory performance may not be possible. Further simplification and miniaturization is needed to make them amenable to mass production with a consequential reduction in unit cost. The art would find much utility in a self-doubling micro-laser which could be manufactured at low cost and which lends itself to mass production.